As known, an optical cable typically comprises an optical core including one or more optical fibers and an external sheath enclosing the optical core. The external sheath is typically made of a polymeric material and has the primary function of protecting the optical core from the mechanical point of view.
Within the external sheath, the optical fibers may be arranged in various ways. In particular, in the so-called “loose tube cables”, the optical fibers are loosely arranged within one or more buffer tubes. Each buffer tube typically contains multiple fibers, and the individual fibers are free to move relative to one another within the buffer tube. In the so-called “central loose tube cables” (briefly, CLT cables), all the optical fibers of the cable are loosely arranged within a single buffer tube, which is in turn enclosed by the external sheath. In the so-called “multi loose tube cables” (briefly, MLT cables), the optical fibers are instead divided into multiple units (e.g. 3, 4 or 6 units), the optical fibers of each unit being loosely arranged within a respective buffer tube. The buffer tubes are then stranded according to an open helix or S-Z arrangement, typically about a central strength member. A binder may also be provided around the buffer tubes for retaining them. Both in CLT cables and in MTL cables, the external sheath may comprise two side strength members (typically made of steel or fiber reinforced resin) embedded within the sheath's thickness and placed at diametrically opposed positions.
Loose tube cables are typically used for applications where the optical fibers must be individually extracted from the cable and spliced, e.g. in FTTH and FTTP applications. For instance, drop cables of FTTH or FTTP networks are typically implemented as CLT or MLT cables with a particularly reduced diameter (less than 10 mm).
In order to extract one or more optical fibers from a loose tube cable and splice them, a length of the external sheath shall be cut and removed from the cable. To this purpose, special tools with blades suitable for making longitudinal cuts in the cable's sheath are known. During the cutting operations, it is desirable avoiding any accidental impact of the blades against the steel strength members embedded within the sheath's thickness. Such impacts may indeed damage the blades, and damaged blades may injury the operator's hands, especially if she/he does not wear protective gloves.
In order to reduce the risk of these accidental impacts, it is known providing grooves on the outer surface of the sheath, which are typically arranged on a longitudinal plane perpendicular to the longitudinal plane containing the steel strength members. Such grooves aid guiding the blades along a path that does not interfere with the steel strength members. However, when the sheath portion to be cut is rather long (few meters or more), deviations of the blades from the path defined by the grooves become likely.
US 2012/0063731 describes an optical cable including a plurality of tight-buffered optical fiber sub-units stranded in a S-Z configuration. A jacket (consisting of one material only) surrounds the sub-units. The exterior of the jacket includes at least two regions of weakness in the form of two parallel longitudinal grooves, namely regions where the thickness of the jacket is less than in the remainder of the jacket. For accessing the sub-units, the jacket is intentionally buckled in the region between the grooves so as to form a rib. The rib is then cut with a tool or with the user's fingernails. Then, the cut edge of the jacket between the grooves is gripped and longitudinally pulled. The grooves serve as a stress concentrator, allowing the strip of jacket material between the grooves to be split from the rest of the jacket and peeled away. The grooves could also be located up to 180° apart from each other on opposed sides of the cable. In this configuration, the installer could remove the jacket in two halves in a “banana peel” fashion, rather than pulling off a single narrow strip. In order to function effectively as a stress concentrator to produce preferential splitting of the jacket, the grooves would have a depth equal to at least 15% of the jacket thickness.